The people who criticize twin and adoption experiments in behavioral genetics don’t seem to realize that using genotyping we’ll be able to test heritability over a continuous range of relatedness, as you emphasize in your post.
Already it is significant that Visscher obtains the same heritability for height that the twin experiments obtained. IQ is next …

I’d like to follow up on the comment made by Agnostic in the “post from the past”: “NorAm Indians, they’re my guess for the ‘living Neanderthals.'” This came on the heels of the Evans et al. (2006) paper that argued for an introgression of MCPH1 haplogroup D from a non-African hominid around 40,000 years ago. The lineage itself diverged 1 MM years ago. Haplogroup D reaches the highest frequency in the Americas. Now we know that, although Lari et al. 2010 identified a different MCPH1 allele in a Neanderthal individual, there are a few other lines of genetic evidence that point to exactly the same pattern: American Indians show world highest frequencies (sometimes close to fixation) of an ancient hominid allele, usually found at low frequencies in modern Africans. Here’s the rundown:
1. X chromosome B006 (the oldest dys 44 lineage, highest frequencies in the New World, lowest frequencies in Africa, worldwide distribution, Neanderthals have it, Yotova et al. 2011).
2. Clade B of Pediculus humanus (2 MM years old, highest frequencies in the Americas, worldwide distribution, lowest frequencies in Africa; archaic introgression suggested by Reed et al. 2004).
3. Blood group O (universal donor blood group, worldwide distribution, highest frequencies in the New World, attested in 2 Neanderthals by Lalueza-Fox).
4. MCPH1 haplogroup D (1 MM years old, highest frequencies in the New World, worldwide distribution, lowest frequencies in Africa, archaic introgression suggested in Evans 2006).
5. mtDNA insert in the nuclear genome (the most divergent human mtDNA sequence, clustered with the Mungo man sequence, highest frequencies in the New World, lowest frequencies in Africa, worldwide distribution; Zischler et al. 1995).
6. mtDNA restriction site morph 1 (modal type, highest frequencies in the New World, worldwide distribution, lowest frequencies in Africa; Johnson et al. 1983).

Coupled with the highest continental levels of linguistic diversity, systematic grammatical archaisms (Nichols 1992), the highest frequencies and the best preservation of most archaic kinship systems (Dziebel 2007), the New World looks like a very old refugium for modern human populations and a likely source for modern human diversity in the Old World.

http://www.kinshipstudies.org German Dziebel

What is also interesting is that the pattern “highest frequency in the New World – lowest frequency in Africa” for these “archaic” alleles mirrors the worldwide distribution of genetic frequencies for several other genetic systems, with Africans and Amerindians being the most widely divergent continental populations. (Razib blogged about it at http://blogs.discovermagazine.com/gnxp/2010/08/which-population-is-most-genetically-distant-from-africans-amerindians/comment-page-1/). The latter pattern is usually explained by a founder effect in the Americas followed by genetic drift affecting small populations. The founder effect would progressively remove most of the diversity accrued by Africans and Asians on the way of becoming Amerindians – making Amerindian homozygosity values the highest in the world, with African homozygosity values staying the lowest -; genetic drift would later divorce the founding Amerindian population into a myriad of highly distinct tribal and village clusters.

This used to be a very convincing – to many but not all scholars – explanation of Amerindian specificity. But in the light of the high frequencies of “archaic” alleles in Amerindians this explanation becomes very problematic. Those “archaic” alleles are found in every corner of the world – from Australia to Western Europe, – and are not specific to Amerindians or to Siberians/East Asians. (Plus it’s not just one case of an archaic allele but a systematic pattern cutting across several genetic systems. We therefore can’t claim that Ameridnians randomly picked up an archaic allele in Siberia.) Consequently, from the point of view of classical out of Africa they must have introgressed into the human gene pool very early on, namely prior to the formation of all of non-Africans. But this would mean that the whole history of non-Africans has been the side effect of the teleological process of breeding Amerindian specificity, which is clearly impossible.

Linguistics comes in handy here, as it suggests that Amerindian populations have been accumulating diversity for a very long time. As a non-biological, non-genetic system, language probably wasn’t affected by diversity suppression mechanisms that have affected Amerindian genetics (long-term drift, isolation, low effective population size, which are in and of themselves features of Lower and Mid-Pleistocene hominid demography) and the diversity increase mechanisms affecting African populations (large effective population size, gene flow across wide distances), and hence linguistic diversity is the best indicator of the patterns of truly neutral evolution in humans.

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com